The present invention relates to a TFT (Thin Film Transistor) array constituting a liquid crystal display (LCD) apparatus and so on.
An active matrix LCD (AM-LCD) using a TFT array in a switching device of a pixel electrode has been well known. This TFT array is configured by arranging plural scanning lines and signal lines so as to intersect at right angles on an insulating substrate and arranging pixel electrodes in the regions surrounded by the scanning lines and the signal lines and providing a TFT (hereinafter called xe2x80x9cfirst TFTxe2x80x9d) for supplying a signal voltage to each pixel electrode. The TFT has a source electrode, a drain electrode, a gate electrode, an active layer, and a gate insulating film. The source electrode is connected to one of the pixel electrodes, and the drain electrode is connected to one of the signal lines, and the gate electrode is connected to one of the scanning lines. When the TFT is selected by a signal from the scanning lines, the TFT conducts and the signal voltage is supplied to the pixel electrodes through the signal lines.
Next, a conventional AM-LCD will be described with reference to the accompanying drawings. FIG. 1 is a plan view showing a configuration of the conventional AM-LCD. As shown in FIG. 1, this AM-LCD is provided with a second TFT 200 for preliminary charging for performing preliminary charging of a pixel electrode 3 other than a first TFT 100 for writing disposed to drive the pixel electrode 3. This second TFT 200 is formed at a diagonal position to the first TFT 100 within a display region. A second gate electrode 21 of the second TFT 200 is connected to a scanning line 1a of the forward row of a scanning line 1b connected to a first gate electrode 11 of the first TFT 100. A second drain electrode 22 of the second TFT 200 is electrically connected to a signal line 2b of the next column of a signal line 2a electrically connected to a first drain electrode 12 of the first TFT 100. Further, a second source electrode 23 of the second TFT 200 is connected to the pixel electrode 3 in a manner similar to a first source electrode 13 of the first TFT 100.
By forming the second TFT 200 in the vicinity of an intersection of the scanning line 1a and the signal line 2b in this manner, even in case that the first TFT 100 is defected, the second TFT 200 instead of the first TFT 100 can supply a signal voltage to the pixel electrode.
Then, a circuit of the TFT array will be described with reference to FIG. 2. As shown in FIG. 2, in the conventional TFT array, a plurality of scanning lines 1a, 1b and 1c are arranged in parallel and a plurality of signal lines 2a, 2b and 2c are arranged perpendicular to these scanning lines. One pixel electrode 3 is arranged in each of a plurality of regions configured by these scanning lines and signal lines.
As a switching device for driving the pixel electrode, a first TFT 100 is formed in the vicinity of an intersection of the scanning line 1b and the signal line 2a, and a first drain electrode 12 of the first TFT 100 for writing is electrically connected to the signal line 2a. 
Further, a gate electrode 11 of the first TFT 100 is connected to the scanning line 1a and a first source electrode 13 of the first TFT 100 is connected to the pixel electrode 3.
However, there was the following problem in the conventional TFT array. The TFT array described above can control occurrence of a point defect etc. by providing two TFTs for driving the pixel electrode.
But, a problem that a decrease in open aperture ratio cannot be ignored in comparison with a TFT array in which only one TFT is arranged in one pixel essentially has occurred. Here, the aperture ratio means a ratio of an area of a portion capable of light modulation to the total area of a pixel. Recently, a decrease in power consumption of the LCD has been desired. For that purpose, it is effective to increase the aperture ratio of the LCD to obtain enough intensity of light. That is, a decrease in aperture ratio has been caused by arranging the two TFTs in an pixel region as a switching device of the pixel electrode.
An object of the present invention is to provide a TFT array for improving writing characteristics into a pixel electrode without a decrease in aperture ratio in order to solve the problem in the conventional art described above.
In order to solve the above problem, a first invention of the present application is characterized in that at least a second TFT for preliminary charging is configured so as to overlap with a part of a region of intersection of a scanning line and a signal line.
By such a configuration, a decrease in aperture ratio of the TFT array can be prevented and as a result, power consumption of a LCD can be decreased.
A second invention of the present application is characterized in that the second TFT for preliminary charging is configured so as to cover a part of a region of forming the signal line.
By such a configuration, even in case that a first TFT 100 is defected, a pixel electrode 3 can be normally driven using a second TFT 200 instead of the first TFT 100, and simultaneously the aperture ratio can be improved.
A TFT array of a third invention of the present application provided to solve the above problem is characterized in that the second TFT for preliminary charging is formed on a scanning line of the forward row of a scanning line connected to a gate electrode of a first TFT for writing.
By such a configuration, a size of the first TFT for writing can be reduced as well as the prevention of a decrease in aperture ratio.
A TFT array of a fourth invention of the present application is characterized in that contact holes extending through a passivation layer are formed on a source electrode of the first TFT for writing and a source electrode of the second TFT for preliminary charging and the two source electrodes are electrically connected to the pixel electrode.
By such a configuration, when forming the TFT array, a formation step of the passivation layer can be eliminated and as a result, yields can be improved and the production costs can be reduced.